Project Summary/Abstract Despite strong conjecture that executive dysfunction is a shared risk factor for both ADHD and Anxiety, and that attentional bias to threat is specific to Anxiety, empirical evidence to explain why the two disorders so commonly co-occur has remained elusive. It is the purpose of this planned study to bridge this gap. We believe that the current lack of progress is due to dependence on the non-specific mean reaction time (RT) to index complex cognition. In comparison, the diffusion model (DM), a computational model of RT, provides a more complete and accurate description of performance because it is based on the shape of the RT distribution for both error and correct responses. During the last grant period, we demonstrated that prominent cognitive hallmarks of ADHD including slow/variable RTs, as well as impairments in executive control, could be traced to slow drift rate, a DM parameter that uniquely indexes the speed at which information accumulates to make a decision. The DM has also been applied to the study of anxiety, identifying a processing advantage for threat- related words, as well as a response bias to threat. In this renewal application, we directly compare the utility of the DM parameters (derived from a gold- standard battery of tasks of executive control and processing biases to threat), against standard RT indices of performance from the same battery. In a sample of 8-12 year old children with ADHD, ANX, and ADHD+ANX, we will determine which set of indicators is more strongly associated with concurrent symptomology, impairment, and neural indices of cognitive function (Aim 1), and which set forms more coherent and informative neurocognitive subtypes (Aim 2). In Aim 3, we determine whether the DM parameters of processing bias to threat are equally capable of capturing and generalizing to performance under conditioned threat, as opposed to the innate threats invoked in Aims 1 and 2. Our goal is to move beyond behavioral descriptions of psychopathology and to identify shared and unique cognitive mechanisms involved in the development of mental health problems, using ADHD and ANX disorders as model systems.